Water electrolyzer

ABSTRACT

An electrolyzer for producing alkaline and/or acidic water by way of electrolysis of water. In order to remove scale such as calcium carbonate deposited on the electrodes of the electrolytic cell (36) during electrolysis, the control unit (166) operates a polarity reversal switch (186) at a predetermined timing to reverse the electric potential applied to the electrodes of the electrolytic cell (36). The control unit (166) includes means (190/192) for detecting the hardness of water and varies the duration of application of DC voltage of opposite polarity in accordance with the hardness of water. Hardness of water is preferably determined by detecting the electric conductivity of water. Time required for removal of scale is shortened.

TECHNICAL FIELD

The present invention relates to a water electrolyzer forelectrochemically producing alkaline and/or acidic water. Moreparticularly, the present invention is concerned with a waterelectrolyzer wherein scales, such as calcium carbonate, deposited onelectrodes during electrolysis are effectively removed.

BACKGROUND ART

It is believed that hydroxyl ion (OH⁻) enriched alkaline water, which isoften incorrectly referred-to as "alkaline ion water", is useful inhealth maintenance when served as potable water as well as inaccentuating taste when used in cooking or for the preparation ofbeverages such as tea and coffee. Similarly, hydrogen ion (H⁺) enrichedacidic water is known as being suitable for boiling noodles and washingfaces. More importantly, highly acidic water which is obtained byelectrolysis of an aqueous solution of sodium chloride and whichtherefore contains effective chlorine (hypochlorous acid or chlorinegas) has been noted as having a strong germicidal effect.

To produce alkaline and/or acidic water, an apparatus for electrolyzingwater has been used hitherto which is often incorrectly referred-to inthe art as "ion water generator". This apparatus, designed to subjectwater to electrolysis, includes an electrolytic cell having an anode anda cathode. As a direct electric potential is applied between theelectrodes, the hydroxyl ions OH⁻ being present in water due toelectrolytic dissociation of water molecules will donate electrons tothe anode at the anode-water interface and are thereby oxidized to formoxygen gas which is then removed away from the system. As a result, theH⁺ concentration is enhanced at the anode-water interface so that H⁺enriched acidic water is resulted at the anode-water interface. At thecathode-water interface, on the other hand, H⁺ accepts electron from thecathode and is reduced to hydrogen to form hydrogen gas which issimilarly eliminated from the system. As a result, the OH⁻ concentrationis increased whereby OH⁻ enriched alkaline water is generated at thecathode side. When an aqueous solution of sodium chloride is subjectedto electrolysis, chlorine gas is generated at the anode and is dissolvedinto water to form hypochlorous acid.

To preclude alkaline water and acidic water once generated byelectrolysis from being mixed with each other and to take them outseparately, the conventional electrolytic cells are typically providedwith a water-impermeable but ion-permeable membrane 3 arranged betweenan anode plate 1 and a cathode plate 2 as schematically shown in FIG. 1,the electrolytic chamber being divided by the membrane into a flowpath 4for alkaline water and a flowpath 5 for acidic water. The electrolyticcell of this type will be referred-to hereinafter as the "membrane-type"electrolytic cell.

As the electrolytic cell is operated, precipitation of scale 6 comprisedof calcium carbonate, calcium hydroxide, magnesium hydroxide and thelike takes place in the flowpath for alkaline water. Referring to FIG. 2wherein the apparent solubility of calcium carbonate versus pH is shown,the mechanism of scale precipitation will be described with reference tocalcium hydroxide by way of an example. It will be noted from the graphthat under acid conditions, calcium carbonate is dissolved into water inthe form of calcium ions. However, as the pH exceeds 8, the solubilityis rapidly drops thereby giving rise to precipitation of calciumcarbonate. In the electrolytic cell of the membrane type, the scaletends to precipitate predominantly on the membrane 3 rather than on thecathode 2, as shown in FIG. 1. Probably, this is because the porousnature of the membrane promotes precipitation of scale, in contrast tothe cathode generally having a polished specular surface. Since theprecipitates such as calcium carbonate are electrically insulating, theelectrical resistance across the cell is increased thereby lowering theefficiency of electrolysis of the cell. In addition, formation of scaleincreases the flow resistance across the electrolytic cell. Therefore,unless the scale is removed, the electrolytic cell would becomeinoperative soon after a short period of use.

Accordingly, there has been proposed in the prior art to remove theprecipitates by dissolving them into water as disclosed, for example, inJapanese Patent Kokai Publication 51-77584, Japanese Utility Model KokaiPublication 55-91996, Japanese Utility Model Kokai Publication59-189871, and Japanese Patent Kokai Publication 1-203097. According tothis method, a polarity reversal switch 7 is turned over in such amanner that an electric potential of an polarity opposite to the normaloperating polarity is applied between the electrodes to thereby causethe precipitates to dissolve. This method of descaling by application ofthe reverse polarity potential is known in the art as "reverse potentialdescaling" or "reverse electrolysis descaling" process. The principle ofreverse electrolysis descaling is that, upon application of electricpotential of the opposite polarity, the flowpath for the alkaline wateris changed into acidic conditions whereby the scale such as calciumcarbonate is disintegrated into ions to again dissolve into water aswill be understood from FIG. 2.

However, since the membrane 3 is more or less spaced from the electrodesas will be understood from FIG. 1, the stream of strongly acidic waterwhich has been generated along the surface of the electrode 2(originally acting as the cathode, but now acting as the anode toproduce acidic water because the polarity of potential is reversed) willbe carried away by the flow of water flowing through the flowpath orwill be diluted by diffusion. Therefore, the membrane which is remotefrom the electrodes cannot be rendered acidic to a degree strong enoughto quickly dissolve the scales deposited on the membrane.

For this reasons, in the "membrane-type" electrolytic cell, it has beendifficult to electrochemically remove the scale even though theso-called reverse electrolysis descaling is carried out. Accordingly, ithas been usual that the life of the electrolytic cells is only from ahalf to one year unless the cells are periodically disassembled and aresubjected to manual mechanical descaling operations. Furthermore, themembrane is unhygienic since it serves as breeding bed for bacteria.

In order to overcome the foregoing disadvantages of the membrane-typeelectrolytic cell, proposed in Japanese Patent Kokai Publication4-284889 is an electrolytic cell which is free from a membrane. Theelectrolytic cell of this type will be referred-to hereinafter as the"non-membrane" type electrolytic cell. In the non-membrane type cell,the electrode plates are spaced from one another with a small gap insuch a manner that a laminar flow is established as water flows betweenthe electrodes. Therefore, alkaline water and acidic water as generatedcan be separated from each other without recourse to a membrane.

As the non-membrane type electrolytic cell is not provided with amembrane which is susceptible to deposition of scale, there is anadvantage that less scale is deposited. The formation of scale takesplace primarily on the electrode plate which acts as the cathode forproducing alkaline water. Moreover, the cell is hygienic because of theabsence of a membrane which would otherwise breed bacteria.

The "non-membrane" electrolytic cell of JP 4-284889 is also designedsuch that the reverse polarity potential is applied to carry out theso-called reverse electrolysis descaling in a manner similar to theconventional membrane-type electrolytic cells.

Regardless of whether the electrolytic cell is of the membrane type orthe non-membrane type, the conventional water electrolyzers are designedsuch that the reverse potential descaling of the electrodes is carriedout at the outset of the period during which water is fed to the cell.In addition, it has been customary to set the duration of the reversepotential descaling constant.

In this regard, the likelihood or tendency of precipitation of thescales such as calcium carbonate varies depending on the quality ofwater which, in turn, significantly differs geographically from regionto region. In a region where the quality of water is such that a largeamount of calcium ions and magnesium ions are contained and, hence, thescales tend to precipitate on the electrodes, it is desirable that theduration of the reverse potential descaling be set for as long period aspossible to ensure that descaling is performed completely. Accordingly,in the prior art, it has been the general practice to set the durationof the reverse potential descaling long enough so as to meet with andcover a region of such an extreme water quality that contains a largeamount of calcium ions and magnesium ions.

However, water that flows out of the electrolyzer during the reversepotential descaling may contain the scales as released from theelectrodes upon application of the reverse potential and, hence, is notsuitable to drink. Therefore, the user who has commenced feed of waterto the electrolytic cell must await and stand by without recovering theelectrolyzed water until the reverse potential descaling is completed.This means that the time interval from the commencement of water feed tothe termination of the reverse potential descaling will amount to theso-called dead time. This brings about the inconvenience that, in aregion where the amount of calcium and magnesium ions is smaller, thestand-by time or wait time would be longer than is necessary so that theelectrolyzer would be difficult to use, if the duration of the reversepotential descaling has been set too long.

The object of the invention is to provide a water electrolyzer whereinthe removal of the scales is effectively carried out so as to extend theservice life of the electrolytic cell and which is yet easy to use inthe sense that the stand-by time required to achieve the reversepotential descaling is minimized.

DISCLOSURE OF THE INVENTION

According to the invention, the water electrolyzer includes anelectrolytic cell provided with a pair of electrodes forming anelectrolytic flowpath therebetween, means for applying between saidelectrodes a DC potential of a predetermined polarity so as toelectrolyze water flowing through said flowpath to thereby producealkaline and acidic water, switching means for reversing the polarity ofthe DC potential applied between said electrodes, and control means forcontrolling said switching means in such a manner that a DC potential ofa polarity opposite to said predetermined polarity is applied betweenthe electrodes at a predetermined timing to remove scale precipitated onthe electrodes during electrolysis of water. The feature of theinvention is that the control means comprises means for detecting thehardness of water to be electrolyzed and is operable to control theduration of application of DC potential of the opposite polarity inaccordance with the hardness of water as detected. The duration ofreverse potential descaling is determined to be longer for hard waterbut shorter for soft water. As in this manner the duration of thereverse potential descaling is varied in accordance with the hardness ofwater, the descaling is carried out only for a requisite minimum timeperiod so that the stand-by time or wait time is shortened.

The hardness of water is proportional to the electric conductivity ofwater. Accordingly, in a preferred embodiment of the invention, theelectric conductivity of water is detected and the hardness of water isderived by presumption from the electric conductivity. Preferably, inorder to detect the electric conductivity of water, the voltage andcurrent of electrolysis applied to the electrolytic cell are detectedand the electric conductivity of water is computed in accordance withthe detected voltage and current of electrolysis. With this arrangement,the hardness of water can be detected in a simple manner whereby thecosts of the apparatus are reduced.

In situations wherein the electrolytic cell is operated alternately inthe alkaline water supply mode and the acidic water supply mode, itwould be ideal to cumulate in a countervailing manner the duration ofelectrolysis in the alkaline water supply mode and the duration ofelectrolysis in the acidic water supply mode so as to detect the modewhich has been used for a longer period of time and to then perform thereverse potential descaling by applying between the electrodes a DCpotential of a polarity opposite to the polarity of the longer-usedmode.

However, in the case where the electrolytic cell is operated firstly inone mode (e.g., the alkaline water supply mode) and then in the othermode (e.g., the acidic water supply mode), the electrodes of the cellwill be subjected to the reverse potential descaling automatically orintrinsically so as to undergo "self-cleaning" because the polarity ofthe potential applied between the electrodes is reversed. It istherefore desirable that the duration of electrolysis in the alkalinewater supply mode and the duration in the acidic water supply mode arecumulated by taking the duration of self-cleaning into account. In thisregard, the time period required for self-cleaning is definite for agiven quality of water so that self-cleaning would not proceed furtheronce self-cleaning has been completed for a predetermined period.Accordingly, it is preferable to cumulate the duration of electrolysisin the respective modes with each other while imposing an upper limitupon them.

The time period necessary for self-cleaning is dependent on the qualityof water, more particularly, the hardness of water. Thus, longer timewill be required for hard water which facilitates precipitation of thescales whereas shorter time will suffice for soft water. Accordingly, inanother preferred embodiment of the invention, the control means derivesa variable which varies in accordance with the hardness of water, thecontrol means being operable to cumulate the duration of electrolysis inalkaline water supply mode and the duration of electrolysis in acidicwater supply mode by countervailing them with each other within a limitof the variable, the control means operating to apply between theelectrodes at a predetermined timing a DC potential of a polarityopposite to the polarity that corresponds to the cumulated duration.With this arrangement, the reverse potential descaling is carried out inan ideal manner.

These features and advantages of the invention as well as other featuresand advantages thereof will become apparent from the followingdescription.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional representation of the conventionalmembrane-type electrolytic cell;

FIG. 2 is a graph showing the apparent solubility of calcium carbonateversus pH;

FIG. 3 is a perspective view showing the mode of use of a waterprocessing unit wherein the water electrolyzer according to theinvention is incorporated;

FIG. 4 is an exploded perspective view of the unit shown in FIG. 3;

FIG. 5 is a cross-sectional view, partly cut away, of an activatedcarbon cartridge of the unit shown in FIG. 4;

FIG. 6 is a cross-sectional view of a temperature-responsive directioncontrol valve of the unit shown in FIG. 4;

FIG. 7 is a perspective view of a temperature sensitive element of thetemperature responsive direction control valve shown in FIG. 6;

FIG. 8 is an exploded perspective view of the electrolytic cell shown inFIG. 4;

FIG. 9 is a cross-sectional view taken along the line IX--IX of FIG. 8and showing the electrolytic cell as assembled;

FIG. 10 is a cross-sectional view taken along the line X--X of FIG. 9,with electrodes and spacers being omitted for simplicity;

FIG. 11 is a cross-sectional view taken along the line XI--XI of FIG. 9;

FIG. 12 is a cross-sectional view taken along the line XII--XII of FIG.9;

FIG. 13 is an enlarged view showing a part encircled by the circle A inFIG. 10;

FIG. 14 is an exploded perspective view of a control valve shown in FIG.4;

FIG. 15 is a perspective view as viewed from below showing a stationarymember and a rotary disk of the control valve shown in FIG. 14;

FIG. 16 is a top plan view of a housing of the control valve shown inFIG. 14;

FIG. 17 is a cross-sectional view taken along the line XVII--XVII ofFIG. 16;

FIG. 18 illustrates an exemplary layout of a control and display panelof the water processing unit;

FIG. 19 is a block diagram of the control unit of the water processingunit;

FIGS. 20A-20C are flowcharts showing the main routine of the controlunit shown in FIG. 19;

FIG. 21 is a flowchart showing a sub-routine for activated carbonregeneration;

FIGS. 22A and 22B are flowcharts showing a sub-routine for the reversepotential descaling of the electrodes;

FIG. 23 is a graph showing the hardness of water versus the electricconductivity of water;

FIG. 24 is a flowchart showing a sub-routine for electrode polaritycontrol and cumulation of the duration of electrolysis; and,

FIG. 25 is a diagram showing the manner in which a cumulation counter isincremented or decremented as the electrolytic cell is operated indifferent modes.

BEST MODE FOR CARRYING OUT THE INVENTION

In FIG. 3, there is shown a water electrolyzer embodying the inventionas incorporated in a water purifier for home use. Referring to FIG. 3,the water processing unit 10 is designed for use as it is placed, forexample, on a kitchen counter 14 equipped with a sink 12. In theillustrated layout, the sink is provided with a mixing valve 16 of thesingle-lever type to which hot water from a boiler (not shown) issupplied via a hot water piping 16A and tap water is applied through awater piping 16B connected to the public water line (not shown).

Spout 18 of the mixing valve 16 is provided with a faucet adapter 20wherein a flow control valve mechanism is arranged, the adapter 20 beingconnected to the processing unit 10 through a tap water supply hose 22and a processed water delivery hose 24. Upon rotating a handle 26 of theadapter 20 into a predetermined angular position, tap water from themixing valve 16 will be forwarded through the supply hose 22 to theprocessing unit 10 and water after treatment will be returned via thedelivery hose 24 to the adapter 20 for delivery through an outlet 28.Upon turning the handle 26 into another position, non processed water ora mixture thereof with hot water coming from the mixing valve 16 will bedirectly delivered from the outlet 28 of the adapter 20 upon bypassingthe processing unit 10. Connected further to the processing unit 10 is adrain hose 30 which is adapted to discharge to the sink 12 waste water,hot water and steam occurred or generated in the processing unit 10.

Referring to FIG. 4, the water processing unit 10 is designed andconstructed such that any particulate matters, such as ferrous rust andmicroorganisms, which are born in tap water are first removed byfiltration, that any harmful or undesirable substances such as residualchlorine, trihalomethanes and smelly substances which are dissolved intap water are then removed under the adsorption action of activatedcarbon, and that the thus purified water is further subjected toelectrolysis to produce acidic and/or alkaline water at the user'sdiscretion. To this end, the water processing unit 10 is comprised of afiltration stage 32 wherein a filter (not shown) such as hollow-fibermembrane filter is received, an adsorption stage including an activatedcarbon cartridge 34 wherein fibrous or granular activated carbon isreceived, and an electrolytic cell 36 for generating acidic or alkalinewater. These components parts of the water processing unit are supportedby a base 40 having a bottom plate 38 and are enclosed by an outercasing 42.

Tap water from the mixing valve 16 is forwarded via the supply hose 22to the filtration stage 32, with filtered water being delivered througha hose 44 to the activated carbon cartridge 34. A conventional flow-ratesensor 46 is arranged at the outlet of the filtration stage 32 to detectthe flow rate of water flowing through the water processing unit 10. Theflow rate sensor 46 is also used to detect the presence or absence ofwater feed to the processing unit 10 and, therefore, may be arranged inany other suitable location or may be replaced by a pressure sensor orpressure switch.

As shown in FIG. 5, the activated carbon cartridge 34 includes acontainer 48 made by curling of stainless steel sheet and is providedwith an inlet 50 to which the hose 44 is connected. Arranged at thecenter of the container 48 is a core frame 52 in the form of a skeleton,around which is fixed an activated carbon element 54 made by moldforming of activated carbon fibers bound by a heat-resistive binder. Tapwater as entered from the inlet 50 into the cartridge 34 will bedistributed over an annular space 56 and will be purified as it flowsacross the element 54 and will be delivered out of an outlet 58 of thecontainer.

The activated carbon cartridge 34 is designed to be heated at a desiredtiming whereupon the activated carbon element 54 is boiled andsterilized, and chlorine and trihalomethanes adsorbed by the element aredesorbed whereby activated carbon is regenerated. To this end, thecartridge 34 is provided with an electric heater 60 fixed to the bottomthereof to ensure that the cartridge 34 is heated at its bottom when theheater is energized. A plurality of screws 62 are welded or soldered tothe bottom plate of the container 48 and the heater 60 is fastenedthereto by nuts. The heater 60 may be comprised of a cup-shaped metallicheat radiator plate 64 and a heat generation member 66 which may be asheathed heater or a mica heater wherein nichrome wires are interleavedbetween mica foils. Preferably, a heat transfer aluminum plate 68 issandwiched between the heated member 66 and the bottom of the containerto ensure that heat of the heated member is conveyed well to thecontainer.

The container 48 has at its center a raised bottom portion to which athermistor 70 is brought into thermal contact in order to detect thetemperature of the raised bottom portion. The thermistor 70 is held intoresilient contact with the central raised bottom portion by a thermistorholder 74 which, in turn, is supported through a coil spring 72 by theheat radiator plate 64. Preferably, an aluminum foil adhesive tape 76 isaffixed over the juncture of the container 48 and the heat radiatorplate 64 to ensure that heat of the heater is transferred well to theactivated carbon cartridge 34.

Referring to FIG. 4, a direction control valve 78 is arranged at theoutlet 58 of the cartridge 34 so as to discharge hot water and steamgenerated during regeneration of activated carbon through the drain hose30 toward the sink 12. The direction control valve 78 is of thetemperature responsive type and is so designed that the outlet thereofis automatically switched over in response to the temperature of hotwater and steam issuing from the cartridge 34. As shown in FIGS. 6 and7, the direction control valve 78 includes a movable member 80incorporating a temperature responsive element made of a thermallyexpansive wax composition, an outlet 82 for purified water and an outlet84 for hot water. The direction control valve 78 is designed such thatin response to a rise in the ambient temperature a spindle 86 isprotruded to move a valve member 88 to the right as viewed in FIG. 6 tothereby permit hot water and steam coming from the cartridge 34 to issuetoward the hot water outlet 84. The direction control valve 78 may beadjusted in such a manner that fluid flow is directed to the purifiedwater outlet 82 when the ambient temperature is less than 60° C. but isforwarded to the hot water outlet 84 when the ambient temperatureexceeds 60° C. As shown in FIG. 4, the purified water outlet 82 of thedirection control valve 78 is connected through a hose 90 to theelectrolytic cell 36, with the hot water outlet 84 being connectedthrough a hot water drainage hose 92 and a T-joint 94 to the drain hose30.

When the water processing unit 10 is operated, purified water issuingfrom the activated carbon cartridge 34 is forwarded to the electrolyticcell 36. Referring to FIGS. 8-13, an embodiment of the electrolytic cell36 will be described by way of an example. In the illustratedembodiment, the electrolytic cell 36 is of the non-membrane type andincludes an elongated pressure-resistive casing 96 made of rigidplastics. As best shown in FIG. 8, the cell 36 is assembled by placing,in sequence, three planar electrodes (i.e., a first lateral electrode98, a central electrode 100 and a second lateral electrode 102) in arecess of the casing 96 with a plurality of plastic spacers 104sandwiched therebetween, followed by fluid tightly fastening a cover 106by screws to the casing 96. Because a pair of lateral electrodes arearranged on both sides of the central electrode 100, the cell 36 of thisembodiment advantageously has a double cell structure. Each of theelectrodes may be made of titanium plate coated with platinum. Aterminal 108 is fixed to each of the electrodes for electricalconnection to a DC power source via an electric cord. In a mode whereinalkaline water is to be produced, an electric potential is applied insuch a polarity that the lateral electrodes 98 and 102 serve as theanode and the central electrode 100 acts as the cathode. In another modewherein acidic water is to be obtained, the electric potential isapplied in the opposite polarity.

As shown in FIG. 9, the casing 96 has an inlet 110 for purified water, afirst outlet 112 for electrolyzed water, and a second outlet 114 forelectrolyzed water, the first outlet operating as the outlet foralkaline water in the alkaline water supply mode but operating as theoutlet for acidic water in the acidic water delivery mode, the secondoutlet serving as the outlet for acidic water in the alkaline waterdelivery mode but serving as the outlet for alkaline water in the acidicwater delivery mode. The inlet 110 is in fluid communication with aplenum chamber or water distribution passage 116 of a generallytriangular cross-section. As best shown in FIG. 10, the plenum chamber116 is defined by the casing 96 and the cover 106 and extends throughoutthe entire vertical length of the electrodes.

As shown enlarged in FIG. 13, a pair of flow paths 118 are formed onboth sides of the central electrode 100. Each of the flow paths concertswith the electrodes to operate as the electrolytic chamber. A pluralityof horizontally extending spacers 104 are sandwiched between theelectrodes to ensure that water coming from the inlet 110 and flowingdown along the plenum chamber 116 flows into the flow paths 118 in thehorizontal direction as shown in FIG. 13. Since the electrode spacing ismade sufficiently small, a laminar flow will be established in the flowof water flowing through the flow paths 118 in the horizontal direction.Accordingly, acidic water and alkaline water which are generatedrespectively along the surfaces of the electrodes by electrolysis can berecovered separately, without providing a membrane between electrodes.

Electrolyzed water produced along the surfaces of the central electrode100 is collected in a first collection passage 120 for electrolyzedwater and is delivered through the first outlet 112. The firstcollection passage 120 is defined by the casing 96 and the cover 106 andextends throughout the entire vertical length of the electrodes in amanner similar to the plenum chamber 116. Electrolyzed water producedalong the surfaces of the lateral electrodes 98 and 102 is recovered insecond collection passages 122 for electrolyzed water. To this end, eachof the lateral electrodes is provided with a slit 124 to ensure that theflow of electrolyzed water flowing along the surfaces of the lateralelectrodes 98 and 102 is directed to flow into the second collectionpassages 122. Electrolyzed water recovered in the second collectionpassages 122 is forwarded to a connection port 126 for delivery from thesecond outlet 114.

Referring again to FIG. 4, a valve unit 128 is connected to the bottomof the electrolytic cell 36 so as to control the direction of two kindsof electrolyzed water (acidic water and alkaline water) flowing out ofthe outlets 112 and 114 of the cell 36. The valve unit 128 may becomprised of a flow control valve 130 and an electric motor 132 with areduction gear mechanism. An example of the flow control valve 130 isshown in FIGS. 14-17.

Referring to FIGS. 14-17, the control valve 130 includes a housing 134,a stationary member 136 positioned within the housing, and a rotary disc140 rotated through a shaft 138 by the motor 132. The housing 134 isprovided with a first inlet 142 connected to the first outlet 112 of theelectrolytic cell 36, a second inlet 144 connected to the second outlet114 of the cell 36, a serviceable water outlet 146, a drain outlet 148,and internal passages therefor. The stationary member 136 and the rotarydisc 140 are formed with various ports and recesses as shown to ensurethat, according to the angular position of the rotary disc 140, theentire amount of water incoming from the outlets 112 and 114 of the cell36 is directed toward the serviceable water outlet 146 or drain outlet148 or, alternatively, electrolyzed water issuing from the first outlet112 of the cell 36 is forwarded to the serviceable water outlet 146while electrolyzed water issuing from the second outlet 114 is deliveredto the drain outlet 148. The serviceable water outlet 146 of the controlvalve 130 is connected to the delivery hose 24 and the drain outlet 148is connected to the drain hose 30.

Referring further to FIG. 4, a control and display section 150 isprovided at the base 40 of the processing unit 10. Also arranged withinthe base 40 is a control unit, described later, which is designed tocontrol the electric heater 60 for regenerating the activated carbon ofthe processing unit 10, the electrolytic cell 36 and the motor 132 ofthe control valve 130. Electric power is supplied to the control unitthrough a cable 152 (FIG. 3).

An example of the layout of the control and display section 150 is shownin FIG. 18. The control and display 150 may include a manualregeneration control switch 154 for commencing regeneration of activatedcarbon in the cartridge 34 in accordance with the instructions of theuser, a regeneration-time-set-mode selection switch 156 to enable theuser either to select a regeneration time which is set by default or tochange the regeneration time set by default, regeneration time setswitches 155 and 157 for incrementing the regeneration time, a liquidcrystal display panel 158, selection switches 160 and 162 to enable theuser to select the kind of water to be delivered, and light emittingdiodes 164 for indicating the selected water.

In the illustrated layout, the control and display section 150 isdesigned such that by operating the selection switch 160 or 162 the usermay select either purified water processed by the filter 32 and theactivated carbon cartridge 34, or acidic or alkaline water obtained bysubjecting purified water further to electrolysis. The pH of alkalinewater may be adjusted in three different levels including strong, mediumand weak. The arrangement may be such that, for example, weakly acidicwater of pH 6.5 is obtained in the acidic water delivery mode, whereasalkaline water of pH 8.5, pH 9.0 or pH 9.5 is obtained in the alkalinewater delivery mode.

In FIG. 19, there is shown an embodiment of the control unit of thewater processing unit 10. An electric power is applied to the controlunit 166 from a commercial AC power source 168 via the cable 152 (FIG.3). The control unit 166 includes a programmed microcomputer 170 whichis programmed in such a manner as to control the power as well as thepolarity of the direct current supplied to the electrolytic cell 36, tocontrol the motor 132 for switching over the destination of waterdelivered from the cell 36, and to control the power supply to theheater 60 intended to regenerate the activated carbon cartridge 34.

The control unit 166 has a diode bridge 172 for full-wave rectifying thealternating current from the power source 168 and a switching powercircuit 174. Briefly, the control unit 166 is designed and constructedsuch that in accordance with various operating parameters themicrocomputer 170 theoretically computes the desired power consumptionof the electrolytic cell 36 and that the microcomputer 170 feed-backcontrols the switching power circuit 174 in such a manner that the poweractually supplied to the cell is equal to the desired power consumption.More specifically, the switching power circuit 174 includes aphotocoupler 176, a capacitor 178 for smoothing the output of thelatter, an integrated circuit 180 having a pulse width modulationfunction, a switching transistor 182 and a switching transformer 184.

The alternating current from the domestic power source 168 is full-waverectified by the diode bridge 172, the DC output of which is applied tothe primary winding of the switching transformer 184. The pulse width ofthe direct current flowing the primary windings of the switchingtransformer 184 is controlled by the switching transistor 182 driven bythe IC 180 in such a manner that an electric current having a wattageproportional to the pulse duty of the primary windings is induced in thesecondary winding of the switching transformer 184. The secondarywinding of the switching transformer 184 is connected to the electrodesof the electrolytic cell 36 through a reversal switch 186 designed toreverse the polarity of the voltage. The reversal switch 186 iscontrolled by a relay 188 which is in turn controlled by themicrocomputer 170. The reversal switch 186 is biased to a position inwhich the central electrode 100 becomes negative when the relay 188 isnot energized and is adapted to be switched over upon energization ofthe relay 188.

A resistor 190 for detecting the intensity of current flowing throughthe cell is connected in series to the lead wires connecting the cell 36and the switching transformer 184, and a pair of resistors 192 fordetecting the voltage applied to the cell are connected in parallel tothe lead wires. The junctions to these resistors 190 and 192 areconnected to input terminals of analog-to-digital converter of themicrocomputer to ensure that the microcomputer 170 periodically checksthe potential at these junctions to detect the intensity and the voltageof electric current supplied to the electrolytic cell.

The control unit 166 further includes a solid state relay (SSR) 194 forcontrolling power supply to the heater 60 for regenerating activatedcarbon, the relay being adapted to be controlled by the microcomputer170. The output signals of the thermistor 70 and the flow-rate sensor 46are sent to the microcomputer 170. The microcomputer 170 furthercontrols the geared motor 132 through a motor driver. A rotary encoderincorporated in the motor detects the rotational angular position of themotor and delivers a corresponding signal to the microcomputer 170. Themicrocomputer 170 controls the motor in accordance with the signals fromthe rotary encoder to, in turn, control the rotary control valve 130whereby the destination of two kinds of electrolyzed water (acidic waterand alkaline water) flowing out of the outlets 112 and 114 of the cell36 is changed over.

The mode of operation as well as the mode of use of the water processingunit 10 will now be described with reference to the operation of themicrocomputer 170 illustrated in the flowcharts shown in FIG. 20A andensuing drawings. Briefly, in the example shown in the flowcharts, themicrocomputer 170 is programmed such that electric power is supplied tothe heater 60 to heat the cartridge 34 to thereby sterilize by boilingand to regenerate the activated carbon everyday automatically wheneverthe predetermined time which is preset by the user for regeneration ofactivated carbon has arrived, as well as each time the user has pressedon the manual regeneration start switch. The time for activated carbonregeneration is set in such a manner that, upon connecting the plug ofthe cable 152 into an associated electrical socket, the time is set bydefault for 13 hours later, for example. The regeneration time thus setby default may be incremented on the one hour basis or on the one minutebasis each time the user presses on the regeneration time set switches155 or 157 after selecting the regeneration time set mode by pressing onthe regeneration-time-set-mode selection switch 156. It is recommendedthat the activated carbon regeneration time is set for midnight in whichit is unlikely that the water processing unit is used.

The microcomputer 170 is also programmed such that, when afterregeneration of the activated carbon, water is fed for the first time inthe morning of the following day, an electric potential of a polarityopposite to the polarity of the mode in which the electrolytic cell isoperated in the preceding day is applied between the electrodes 100 and98/102 of the cell 36 to remove the scales such as calcium carbonateprecipitated on the electrodes. According to the invention, the durationof the reverse potential descaling is varied in response to the hardnessof tap water.

More specifically, referring to the flowcharts shown in FIG. 20A andensuing drawings, upon engaging the plug of the cable 152 with thesocket, the microcomputer 170 is initialized (S201) and the motor 132 isreturned to its initial position in which the two outlets 112 and 114 ofthe electrolytic cell 36 are connected to the drain hose 30 (S202). Inthe microcomputer initializing sequence (S201), a "regeneration flag"indicative of a demand for activated carbon regeneration (see S211) andan "electric conductivity calculation flag" (S406) indicative ofcompletion of computation of the electric conductivity are resetrespectively to "0". In this regard, these flags as well as variousother flags, counters and timers described later may be implemented bythe microcomputer 170 and its associated memories. A "reverse potentialdescaling flag" is then set to "1" (S203). The reverse potentialdescaling flag "1" is intended to mean a demand for the reversepotential descaling while the reverse potential descaling flag "0"indicates that reverse potential descaling need not be carried out.

Then, a suitable number is preliminarily input into a cumulation counter(S204). In this regard, the cumulation counter is intended to sum theduration of electrolysis in the alkaline water delivery mode and theduration of electrolysis in the acidic water delivery mode whilecountervailing them with each other and is adapted to be incrementedwhen electrolysis is conducted in the alkaline water delivery mode butto be decremented in the acidic water delivery mode, as will bedescribed later in more detail with reference to the flowchart of FIG.24. At S204, a suitable number (say, 1) is input into the cumulationcounter in order to permit measurement of the hardness of water uponcommencement of water feed. Then, an initial measurement demand flag,indicating that an initial measurement of the hardness of water isrequested, is preliminarily set for "1" (S205) and then S207 andfollowing tasks are repeated, for example, for every 10 milliseconds(S206).

First, the outputs from the thermistor 70, flow rate sensor 46, currentdetection resistor 190 and voltage detection resistors 192 are input andthe flow rate is computed based on the output from the flow rate sensor46 (S207), followed by reading of the switch inputs of the control anddisplay section 150 (S208) and energization of the display panel (S209).

When after connection of the cable plug the mixing valve 16 is opened tofeed water to the processing unit 10, tap water will be purified by thefilter 32 and the activated carbon cartridge 34 whereby purified wateris forwarded to the electrolytic cell 36. As water feed is sensed inaccordance with the signals from the flow rate sensor 46 (S210), thereverse potential descaling flag is checked (S217). Since this flag hasalready been set to "1" at S203 as mentioned before, a sub-routinesequence for "reverse potential descaling" of electrodes is firstcarried out upon commencement of water feed (S218) whereby measurementof the electric conductivity of water is performed.

The detail of the reverse potential descaling sub-routine is shown inthe flowchart of FIGS. 22A and 22B. Referring to FIGS. 22A and 22B, thevalue of the cumulation counter is first checked (S401). Because thecumulation counter has previously been input with "1" at S204 asmentioned before, the polarity reversal relay 188 is energized (S403)upon commencement of water feed. Then the electric power to be suppliedto the electrolytic cell 36 for electrolysis is set, for example, to 30W (S404) and the power supply is controlled such that the electrolyticpower of the cell 36 becomes 30 W (S405). Thus, by way of theproportional-plus-integral-plus-derivative (PID) control method, asignal having a pulse width corresponding to a wattage of 30 W isdelivered from the power control terminal of the microcomputer to thephotocoupler 176 and the switching power circuit 174 is feed-backcontrolled in such a manner that the electric power actually supplied tothe electrolytic cell is equal to 30 W. In this manner, power supply tothe anode and cathode of the cell 36 is started to in turn commence theelectrolysis of water.

Then, the electric conductivity calculation flag is checked to see ifthe calculation of the electric conductivity has been completed (S406).As this flag has been reset to zero by the initialization (S202) of themicrocomputer as mentioned before, then task S407 is performed tocalculate the electric conductivity of tap water. To this end, theelectric potential at the junctions to the current detection resistor190 and voltage detection resistors 192 is monitored so as to detect thecurrent intensity I flowing across the cell electrodes as well as thevoltage V applied between the electrodes and then, based on the currentintensity I and the voltage V thus detected, calculation of the electricconductivity κ may be carried out in the following manner. First, thecoefficient of conductivity χ is computed. The coefficient ofconductivity X is expressed by the following equation.

    χ=S·d/A                                       (1)

where S is the conductance of water, d is the electrode spacing, and Ais the surface area of the electrodes.

As S=I/(V-V₀) where V₀ represents the hydrogen overvoltage, bysubstituting it into equation (1) above,

    χ=I·d/{(V-V.sub.0) A}                         (2)

Equation (2) may be rewritten as follows.

    χ=K·I/(V-V.sub.0)                             (3)

where K is a constant.

Since the coefficient of conductivity χ of water corresponds insubstance to the electric conductivity of water κ(χ=κ),

    κ=K·I/(V-V.sub.0)                           (4)

When the electric conductivity is calculated in the foregoing manner, anelectric conductivity calculation flag is set to indicate completion ofthe calculation of the electric conductivity (S408) and the duration Tof reverse potential descaling is calculated in accordance with theelectric conductivity (S409). The duration T of reverse potentialdescaling may be determined to be T=Cκ, where C is a constant and may be1/10, for example. As shown in the graph of FIG. 23 wherein statisticaldata of various public waterworks bureaus are given, the hardness ofwater is substantially proportional to the electric conductivity ofwater so that the electric conductivity is increased with increasinghardness. Accordingly, by determining the duration T of reversepotential descaling in accordance with the electric conductivity ofwater, the duration T would be fixed longer for water of such a qualitythat contains a large amount of calcium and magnesium ions and,therefore, precipitation of the scales is promoted. Conversely, forwater of a quality having a small hardness, the duration of reversepotential descaling is shortened whereby the wait time of descaling canbe curtailed.

Thereafter, the reverse potential descaling duration T calculated inthis manner is set as the upper and lower limits of the cumulationcounter (S410) and then the initial measurement demand flag is checked(S411). As this flag has previously been set to "1" (S205), then thereverse potential descaling duration is once set to zero (S412) and theinitial measurement demand flag and the reverse potential descaling flagare cleared (S413 to S414) before returning to the main routine.

As the descaling flag is cleared (S414) in this manner after calculationof the electric conductivity as well as the reverse potential descalingtime has been completed upon commencement of water feed, the next cycleof the main routine shown in FIGS. 20A-20C is so conducted that, afterdecision at S217, task S219 is performed wherein the control valve 130is rotated to the water delivery position in which the first outlet 112of the cell 36 is connected to the delivery hose 24. Then, a desiredelectrolysis power required to produce water having a pH appointed bythe user through the switch 160 or 162 is computed (S220) and theelectric power is controlled toward the desired electrolysis power(S221) to commence delivery of electrolyzed water.

During electrolysis, the electrode polarity is controlled and theduration of electrolysis in alkaline water supply mode and the durationof electrolysis in acidic water supply mode are cumulated while beingcountervailed with each other (S222). The detail of the polarity controland cumulation (S222) is shown in the flowchart of FIG. 24. First, basedon the switch inputs, decision is made to see whether the user hasselected acidic or alkaline water (S501).

When the user has selected "acidic water", the relay 188 is energized(S502) to switch over the reversal switch 186 so that an electric poweris supplied to the cell 36 in such a polarity that the central electrode100 acts as the anode and the lateral electrodes 98 and 102 serve as thecathode. Accordingly, acidic water is obtained at the first outlet 112of the cell 36 and alkaline water is delivered from the second outlet114. Acidic water is sent to the delivery hose 24 while alkaline wateris forwarded to the drain hose 30. During operation in the acidic waterdelivery mode, the cumulation counter is decremented on the one secondbasis (S503).

When conversely the alkaline water supply mode is selected, the relay188 is de-energized (S506) so that an electric power is supplied to thecell 36 in such a polarity that the central electrode 100 acts as thecathode and the lateral electrodes 98 and 102 serve as the anode. As aresult, alkaline water is generated along the surfaces of the cathodeand is forwarded to the first outlet 112 of the cell 36, with acidicwater being produced along the surfaces of the anodes and forwarded tothe second outlet 114. Alkaline water thus produced is sent via thedelivery hose 24 to the faucet spout whereas acidic water is forwardedto the sink through the drain hose 30. It will be noted that, ifstrongly alkaline water is selected in the alkaline water delivery mode,correspondingly strongly acidic water will be obtained at the secondoutlet 114. Such strongly acidic water may be recovered from hose 30 andmay be used for the purposes of sterilization and the like. Duringoperation in the alkaline water delivery mode, the cumulation counter isincremented on the one second basis (S507).

An upper and lower limit are provided in cumulation of the duration ofelectrolysis (S504-S505; S508-S509) to ensure that the cumulated timedoes not surpass a limited range. This limit is a variable which variesin accordance with the water quality (i.e., the hardness of water) andmay be identical in number to the duration of reverse potentialdescaling (S410) as described before.

The manner of cumulation of the duration of electrolysis will bedescribed with reference to the graph of FIG. 25 wherein three patternsof cumulation are illustrated. If the electrolytic cell is operated inthe alkaline water supply mode, the duration of electrolysis will beincremented, for example, in the positive as shown in the left part ofFIG. 25. If alkaline water delivery mode is followed by acidic waterdelivery mode as shown in the central part of the graph, the duration ofelectrolysis in the alkaline water supply mode and the duration ofelectrolysis in the acidic water supply mode will be countervailed witheach other so that the cumulated time will be of the negative value ifthe duration of electrolysis in the acidic water delivery mode islonger. When the duration of operation in the alkaline water supply modereaches the upper limit, for example, as shown in the right part of thegraph of FIG. 25, the cumulated time will be limited to the upper limitX. The same applies to the lower limit -X. In this manner, when operatedin both the alkaline water and acidic water delivery modes, the durationof electrolysis in respective modes will be cumulated within the limitof the maximum value X.

When the use of the water processing unit 10 is terminated and as longas water feed thereto is stopped (S210), the activated carbonregeneration flag is checked (S211). Since this flag has preliminarilybeen reset to zero in the microcomputer initializing sequence, theprogram skips to S213 to check whether the time for activated carbonregeneration has arrived. After the use of the processing unit 10 for agiven day has ended and upon arrival of the activated carbonregeneration time (S213) which has been preset for midnight by theregeneration time set switches 155 and 157 of the control board 150, thereverse potential descaling flag is set (S214), the activated carbonregeneration flag is set to "1" (S215) and the control valve 130 isrotated to the drain position (S216) so as to connect the both outletsof the cell 36 to the drain hose 30. As the activated carbonregeneration flag is now set to "1" (see S215), the result of thedecision at S211 in the next cycle of the program shown in FIGS. 20A-20Cwill be "YES" so that heating and regeneration of the activated carbonwill be commenced (S212).

Referring to FIG. 21 wherein the sub-routine for the activated carbonregeneration (S212) is shown, a heating completion flag is first checked(S301). In this regard, the heating completion flag is used to indicatewhether or not heating of the activated carbon cartridge 34 has beencompleted and this flag is intended to distinguish a sequence (S304) inwhich the temperature rise of the cartridge 34 is monitored from asequence (S307) in which the temperature drop is monitored as describedlater.

Since the heating completion flag has preliminarily been reset to zeroduring the course of the initialization of the microcomputer (S201), theSSR 194 is energized to commence power supply to the electric heater 60(S302). Then, a warning message reading, for example, as "underregeneration" or "under preparation" is displayed on the display panel158 (S303) to preclude the user from inadvertently using the waterprocessing unit.

Upon operation of the heater, the activated carbon cartridge 34 isheated causing water in the cartridge 34 to boil. As the heater 60 isoperated, the wax element of the temperature-sensitive direction controlvalve 78 expands to switch over the valve 78 whereby hot water and steamgenerated in the cartridge 34 are discharged through the drain hose 30into the sink 12. Under the action of hot water and steam, the activatedcarbon in the cartridge is sterilized by boiling and chlorine ions andvolatile substances such as trihalomethanes which have been adsorbed bythe activated carbon are desorbed from the activated carbon whereby theactivated carbon is regenerated.

Regeneration of the activated carbon will be completed when watercontained in the cartridge 34 and water impregnated in the activatedcarbon are depleted by evaporation, whereupon the temperature of thecartridge 34 will begin to rise. When the temperature as detected by theoutput signal of the thermistor 70 has exceeded, say, 120° C. (S304),power supply to the heater 60 is terminated (S305).

Then, the heating completion flag is set to "1" (S306) to ensure thatcooling of the cartridge 34 is monitored (S301→S307). As the cartridge34 is cooled by heat radiation and the ambient temperature of thetemperature responsive valve 78 becomes lower than 60° C., the valve 78responds automatically to connect the outlet 58 of the cartridge 34 tothe water delivery hose 24 so as to bring the processing unit in acondition ready for use. As the temperature of the cartridge 34 isfurther lowered to become lower than 40° C. (S307), a "ready" message isdisplayed (S308) and the heating completion flag and the regenerationflag are reset to "0" (S309-S310).

After regeneration of the activated carbon is automatically commencedupon arrival of the preset time, preferably at night, and is thencompleted in the foregoing manner, tasks S206-S211 will be repeateduntil water is fed for the first time in the next morning. Since thedescaling flag has already been set (S214) upon arrival of the activatedcarbon regeneration time, the program proceeds from S217 to S218 at thetime of the first water feed in the next morning (S210) whereby thereverse potential descaling of the electrodes is carried out.

Referring to the flowchart of FIGS. 22A and 22B, in the subroutine forthe reverse potential descaling, the cumulation counter is checked(S401) and the relay 188 is controlled in such a manner that an electricpotential of a polarity opposite to the polarity of the mode which hasbeen used for a longer period in the preceding day is applied betweenthe electrodes (S402 or S403), followed by setting of the electric power(S404) and control of the power supply (s405) as described before. Inthis regard, in the case where the sum of the cumulation counter iszero, the program skips to S416 to thereby omit the reverse potentialdescaling, in consideration of the fact that, because the alkaline waterdelivery mode and the acidic water delivery mode have been alternatelyperformed for an equal period of time, the scales such as calciumcarbonate which might have precipitated on the electrodes have alreadybeen removed so that the electrodes have been self-cleaned.

It will be noted that, at the time of the first descaling after thewater processing unit 10 has been connected to the power source, theinitial measurement of the electric conductivity has been completed asdescribed before so that the electric conductivity calculation flag hasalready been set (S408). Accordingly, task S406 is followed by task S415whereby the reverse potential descaling will be carried out until thedescaling time determined in accordance with the electric conductivityelapses.

At the time of the reverse potential descaling carried out more than oneday after connection to the power source, task S406 is followed by taskS407 whereby the electric conductivity is measured and renewed everyday.Upon completion of the reverse potential descaling in the foregoingmanner, the cumulation counter, the reverse potential descaling flag andthe electric conductivity calculation flag are cleared, respectively(S416-S418). As a result, as long as electrolysis of water is continued,the computation of the desired electrolysis power (S220) and the powercontrol (S221) are performed and the duration of electrolysis iscumulated (S222). It will be noted that, because the initial measurementdemand flag (S411) has been cleared (S413) at the time of the firstdescaling after power source connection, the second and subsequentdescaling after power source connection will be carried out only for thetime period calculated at S409.

While the present invention has been described herein with reference tothe specific embodiments thereof, it is contemplated that the inventionis not limited thereby. For example, although the reverse potentialdescaling of the electrodes for the removal of the scales has beendescribed as being carried out once everyday, the cycle thereof may bealtered as required. Similarly, while the water electrolyzer accordingto the invention has been described as being incorporated in a waterpurifier, the electrolyzer may be used solely. Furthermore, anelectrolytic cell of the membrane type may be used in lieu of thenon-membrane type cell.

We claim:
 1. In a water electrolyzer having an electrolytic cellprovided with a pair of electrodes forming an electrolytic flowpaththerebetween, means for applying between said electrodes a DC potentialof a first polarity to electrolyze water flowing through said flowpathand thereby produce alkaline and acidic water, switching means forreversing the polarity of the DC potential applied between saidelectrodes, and control means for controlling said switching means insuch a manner that a DC potential of a polarity opposite to said firstpolarity is applied between the electrodes to remove scale precipitatedon the electrodes during electrolysis of water;wherein the improvementto said control means comprises means for detecting the hardness ofwater to be electrolyzed and wherein said control means is adapted tocontrol the duration of application of DC potential of said oppositepolarity in accordance with the detected hardness of water.
 2. A waterelectrolyzer according to claim 1, wherein said means for detecting thehardness of water comprises means for detecting the electricconductivity of water and wherein said control means is operable todetect the hardness of water in accordance with the detected electricconductivity of water.
 3. A water electrolyzer according to claim 2,wherein said means for detecting the electric conductivity of water isoperable to detect the voltage of the potential applied between saidelectrodes and the current flowing across said electrodes and whereinsaid control means is operable to detect the electric conductivity ofwater in accordance with the detected voltage and current.
 4. A waterelectrolyzer according to claim 3, wherein said control means derives avariable which varies in accordance with the hardness of water, saidcontrol means being operable to cumulate the duration of electrolysis inalkaline water supply mode and the duration of electrolysis in acidicwater supply mode by countervailing them with each other within a limitof said variable, and wherein said control means is operable to applybetween the electrodes a DC potential of a polarity opposite to thepolarity that corresponds to the cumulated duration.
 5. A waterelectrolyzer according to claim 2, wherein said control means derives avariable which varies in accordance with the hardness of water, saidcontrol means being operable to cumulate the duration of electrolysis inalkaline water supply mode and the duration of electrolysis in acidicwater supply mode by countervailing them with each other within a limitof said variable, and wherein said control means is operable to applybetween the electrodes a DC potential of a polarity opposite to thepolarity that corresponds to the cumulated duration.
 6. A waterelectrolyzer according to claim 1, wherein said electrolytic cell is ofthe non-membrane type.
 7. A water electrolyzer according to claim 6,wherein said control means derives a variable which varies in accordancewith the hardness of water, said control means being operable tocumulate the duration of electrolysis in alkaline water supply mode andthe duration of electrolysis in acidic water supply mode bycountervailing them with each other within a limit of said variable, andwherein said control means is operable to apply between the electrodes aDC potential of a polarity opposite to the polarity that corresponds tothe cumulated duration.
 8. A water electrolyzer according to claim 1,wherein said control means derives a variable which varies in accordancewith the hardness of water, said control means being operable tocumulate the duration of electrolysis in alkaline water supply mode andthe duration of electrolysis in acidic water supply mode bycountervailing them with each other within a limit of said variable, andwherein said control means is operable to apply between the electrodes aDC potential of a polarity opposite to the polarity that corresponds tothe cumulated duration.